Typically, a solid state laser cavity contains a host material that is doped with a small amount of an activator ion. This ion can be pumped by a light source such as a flash lamp or more commonly, a diode laser of suitable frequency. The light from the pump is absorbed by the gain medium, i.e., the doped host, creating a population inversion that causes stimulated emission of coherent light. The output light can be in the form of continuous or pulsed emission.
While the gain medium can be the only crystal regime of a laser cavity, solid-state lasers often employ several single crystal regimes that serve a series of purposes. Typically these regimes occur in the form of a series of layers or films that have similar lattice structures and dimensions, but with slightly different chemical compositions where the different compositions reflect the different functions. For instance, some applications require a high-powered pulse. This can be achieved using relatively low pump power and a Q-switch employed in conjunction with the gain medium. Accordingly, short pulses of high power emissions using high frequency pulse modes can be provided.
Techniques of Q-switching encompass either active or passive methods. The active method provides excellent control but is quite complex. The passive mode is far simpler and requires only a suitable saturable absorber material. The absorber material is a solid-state host doped with a minor amount of an ion (i.e., a Q-switch ion) that serves as passive trigger and is usually tailored to match the output light of the particular activator ion of the laser. Once light saturates the absorber, transmittance through the saturable absorber material rapidly increases (often referred to as “bleaching”) and the absorbed energy is emitted from the laser cavity as a pulse of short duration with a high peak power.
This system is particularly suitable for diode pumped solid-state microlasers. A typical design employs a crystal doped with a suitable activator ion pumped with a modest powered diode laser and capable of emitting between about 10 and 100 mW. Generally the laser cavity is very small, on the order of 1 mm. This requires only a minimal power supply and small diode pump laser. To obtain short bursts of higher peak powers of several kW for a few nanoseconds a passive saturable absorber Q-switch is utilized in conjunction with the emission material. Passive saturable absorber Q-switches have been previously described (see, e.g., U.S. Pat. No. 5,394,413 to Zayhowski). Attempts have been made to improve efficiency of such processes by minimizing optical loss between the emission of the gain medium and the absorption of the saturable absorber, for instance through utilization of certain dopant combinations such as Nd3+ and Cr4+ (see, e.g., Okhrimchuk, et al., Opt. Mater. 3 (1994) 1-13; Degnan, IEEE J. Quantum Electron. 31 (1995) 1890-1901; U.S. Pat. No. 5,119,382 to Kennedy, et al.).
Additional crystal regimes can be incorporated into a laser cavity including those to obtain thermal management, mechanical strength, waveguiding capabilities, and the like. Thermal management is a significant matter particularly with regard to high-energy solid-state lasers. For instance, thermal lensing and surface distortion become a significant matter for higher power applications during which residual heat buildup can be localized in the active lasing cavity in a non-uniform fashion. This leads to irregular thermal expansion of the lattice and reduces the quality of the beam. Many of these issues have been summarized in the technical literature (for example Armstrong et al. Optics Comm. 2000, 175, 201; McDonald et al. Optics Comm. 2000, 178, 383), as well as the patent literature (see, e.g., U.S. Pat. No. 6,845,111 to Sumida, et al., U.S. Pat. No. 6,944,196 to Wittrock, and U.S. Pat. No. 5,761,233 to Bruesselbach, et al.). In thin disk lasers it is often desirable to have a thin layer region doped with activator ions on a thicker undoped region that serves a supporting substrate. Such devices have been described in e.g., U.S. Pat. No. 6,347,109 to Beach, et al, and U.S. Pat. No. 6,834,070 to L. E. Zapata.
In one design both the activator solid and the secondary function material, e.g., the Q-switch, are based on the same host and only differ in the particular ion doped into the host lattice. Since the dopant ion is often similar in size to the other metal ions in the lattice and is only present in small amounts (usually between 0.1 and 10%), the lattice size and structure is mostly unaffected.
In many applications the different regions are relatively small. For example a gain medium can be between about 0.5 and about 1.0 mm in thickness, while a Q-switching region can be between about 0.03 and about 0.5 mm in thickness. In addition to crystal phases, the crystal surfaces can be coated with multiple dielectric films to control reflection and absorption, but these add little to the overall length of the laser cavity. Small size of the components can reduce the pulse width and lead to a very small, simple laser cavity that provides an output beam having a high repetition rate of high peak powers with short duration and single mode well-shaped pulses. These small, simple, rugged devices are called microlasers and have the advantages of modest input power but brief coherent high peak output. Microlasers are useful for various applications including range finding, optical communication, micromachining, environmental monitoring and many other applications (see, e.g., Zayhowski, Opt. Mater. 11 (1999) 255-267; Zayhowski, Laser Focus World, August 1999).
To create a solid-state laser cavity with multifunctionality, it is necessary to produce the various regions attached to one other through a robust and precise bonding mechanism. Construction is complicated by the need for very precise control of the concentration dopant ion and thickness of the layers. Furthermore, if the resultant output beam is to be frequency manipulated through a non-linear process (for example second harmonic generation or optical parametric oscillation), it is useful to have an output with controlled polarization. All of these requirements lead to the need for very exact control over the various layers in a solid-state device.
There are two general techniques presently in use to form multifunctional crystal devices. One method is direct bonding of different premade materials. Use of glues, fluxes or other bonding materials has been examined but is usually unacceptable due to degradation of the optical beam quality. Other direct bonding methods include pressure bonding, electrical potential fusion and other techniques, but these are often expensive, unreliable or otherwise not practical for scalable production of layers between 50-1000 microns (μm). Bonding methods have been described in, e.g., U.S. Pat. Nos. 5,441,803, 5,563,899, 5,846,638, 6,025,060 and U.S. Patent Application Publication No. 2009/0041067.
A second method has been the growth of layers directly on a suitable substrate to form a monolithic composite. Typically this has been accomplished through epitaxial growth in which one material acts as a substrate and a second material is deposited on the surface in a stepwise controlled manner. The grown layer adopts the general structural characteristics of the substrate (such as same lattice type and similar dimensions). Generally this process requires that the two materials have a similar structure type and reasonable crystal lattice match. In the case of solid-state laser devices, the use of gas phase epitaxial methods (molecular beam epitaxy, physical vapor deposition, MOCVD etc.) have not been suitable as gas phase methods are too slow to form the desired layer thickness (such as 0.1-1 mm).
Liquid phase epitaxy (LPE) as described by B. Ferrand, et al. (see, e.g., Opt. Mater. 11 (1999) 101-114; U.S. Pat. No. 6,973,115; EP Patent No, EP-A-0 653-82) has also been used to grow layers of suitable thickness. LPE employs high temperature fluxes to dissolve the substrate material and deposit the appropriate layers on the substrate seed via supersaturation. It typically employs molten salts that are usually mixtures of lead oxide and boron oxide or other metal oxides that melt between 1200° C. and 1600° C. and impart modest solubility to the desired layer material. Unfortunately, the LPE method often utilizes highly toxic lead-based solvents and requires very high temperature processing, leading to increased environmental danger and costs. Additionally, the formed boule must be treated to spin away flux and cleaned with nitric acid to remove any residual flux. Furthermore, the high temperature solvents often contaminate the resultant product with the flux and/or impurities in the flux.
Hydrothermal techniques, in which a temperature differential is developed to create a supersaturated solution leading to crystal growth on a seed, have been utilized for bulk single crystal growth (see, e.g., R. A. Laudise, J. W. Nielson, Solid State Phys. 12 (1961) 149-222), but are not well known for use in forming heterogeneous materials. For example electronic grade quartz is grown commercially by the hydrothermal method. Other crystals, such as potassium titanyl phosphate (KTP) are grown by both flux and hydrothermal methods, and it is widely acknowledged by those familiar with the art that the hydrothermally grown products are of generally superior quality.
What are needed in the art are methods for forming solid state laser devices incorporating multiple crystal regimes that are more economical than previous methods. For example, a low temperature, facile process that can provide a monolithic heterogeneous crystal including a Q-switch for use in a laser cavity would be of great benefit.